Numerous attempts have been made to provide an ignition system with some form of advance at higher engine speeds for use in engines such as are used on snowmobiles, outboard motors, and the like, where improved performance at higher engine speed is desirable, and fuel consumption and duration of operation make improvements in fuel economy desirable. Some such systems have been mechanical, and provide an ignition advance proportional to engine speed, or to throttle position, many have borrowed from the historical technique for measuring speed of a rotating device, using a magnetic pickup sensing the passage of gear teeth or the passage of a projection on a shaft, to generate a voltage related to the rotational speed of the gear or shaft. Rotating discs have been provided with projecting ramps tapering in both width and height, square and round pins or various heights and lengths protruding from the rim or face of a rotating member, and other configurations, complex to manufacture, cumbersome to assemble and expensive to produce.
U.S. Pat. No. 3,619,634 is exemplary of such attempts. U.S. Pat. No. 3,619,634, issued to Bob O. Burson on Nov. 9, 1971, entitled "ALTERNATOR AND COMBINED BREAKERLESS IGNITION SYSTEM" discloses a capacitive discharge ignition system for an engine, the triggering of the ignition system to fire the associated spark plug being achieved by a magnetic pickup, sensing irregularities such as pins on the outer surface of the rim of the rotor. The irregularities take the form of a ramp, non-linearly tapered in height and width, and which may be proceeded by a rectangular projection, a series of rectangular projections or ribs of equal height and unequal width, as well as series or recesses of decreasing size formed in the outer rim surface. Such rotors, used with conventional magnetic pickup, are more difficult to manufacture than gears, for the reasons that the teeth, recesses or projections are not identical, and require complex machining operations to provide the accurate surfaces necessary to provide repeatable air gaps.
U.S. Pat. No. 3,961,618, issued to Swift on June 8, 1976, entitled "SMALL ENGINE IGNITION SYSTEM WITH SPARK ADVANCE", assigned to the instant assignee, discloses the use of an eccentric magnetic rotor, with a radial slot to accentuate flux reversal, and a radial notch spaced from the radial slot for providing an advance trigger signal. The radial notch is of complex shape, having a sloped trailing edge, apparently to prevent the generation of a pulse by the trailing edge, which could result in false triggering.
The instant overcomes numerous deficiencies of prior attempts to provide similar results.